Medical X-ray transparencies are usually examined by placing them over the viewing surface of a device commonly referred to as an illuminator, light-box or viewbox. Conventional illuminators normally comprise a box-like structure enclosing a plurality of fluorescent or incandescent lighting tubes behind a light diffusing plate which defines the display area. Generally, transparencies are retained on the surface of the viewing surface by pushing the upper edge of the transparencies under spring-loaded film-holder clips located along the top edge of the viewing surface.
Standard size illuminators have a viewing surface 17 inches high and 14 inches or multiples of 14 inches (i.e. 28 inches or 56 inches) wide. Usually, each 14 inch width of viewing surface has its own fluorescent tubes and control switch. Such viewing surface enables the viewing of standard size X-ray films which measure up to 17 inches by 14 inches.
The sections of the viewing surface not covered by transparencies need not be illuminated. This eliminates unnecessary glare from areas outside the transparency. When transparencies smaller than 14 inches by 17 inches are to be examined, they are typically retained on the display area in the same manner as full size transparencies, for example, by suspending them by means of the film-holders along the top of the viewer. This leaves a portion of the display area surrounding the transparencies fully illuminated, with the resulting glare detracting from the visual perception of the person trying to study the transparency and assess the information it contains.
Often transparencies contain several very transparent areas, and, frequently, radiologists have to examine transparencies which over-exposed (i.e., very dark) in some portions and under-exposed (i.e., very light) in others. In these cases, considerable glare emanates through areas of the transparencies themselves. Moreover, in many instances, the region of the display area which is of highest interest to the viewer is quite dense. Thus, the ability to discern details in the region may be limited by glare from the surrounding less dense areas.
Attempts have been made in the past to provide viewing devices for X-ray transparencies which shield the eyes of the observer from light other than that passing through the transparencies. These viewing devices obscure light from portions of the transparencies not of interest or from outside the transparencies and/or reduce the contrast within transparencies.
U.S. Pat. No. 1,988,654 to Haag discloses a light box which incorporates two manually movable curtains for masking all of the light-transmitting surfaces of a diffuser up to the edges of a transparency.
U.S. Pat. No. 2,436,162 to Cadenas discloses an X-ray viewer having a masking arrangement incorporating a plurality of hinge--connected opaque masks which may be manually pivoted relative to each other to expose all or only selected parts of an X-ray transparency.
U.S. Pat. No. 4,004,360 to Hammond is directed to a self-masking viewing device which purports to automatically obscure areas of the viewing screen not occupied by the X-ray transparency. In the disclosed device, the screen is provided with a multiplicity of holes which may be selectively blocked by shutters or opened for the passage of light. The interior of the device is connected to a vacuum source which functions to hold the film against the front surface of the device.
The vacuum functions, in addition, to close the shutters connected with those holes not covered by the transparencies, so that passage of light through such holes is prevented. Air cannot pass through those holes in registry with the transparencies and, thus, the shutters associated with the covered holes remain open for the passage of light. The device described is unsuitable for critical inspection of X-ray transparencies since the presence of holes and shutters in the areas in registry with the transparencies creates a pattern behind the transparencies which interferes with the ability to accurately read them.
U.S. Pat. No. 4,373,280 to Armfield discloses an X-ray viewing plate having a cross bar for supporting transparencies at a central portion of the screen. A series of shades is provided which may be manually activated to obscure selected parts of the illuminated surface.
U.S. Pat. No. 4,510,708 to Porkinchak discloses an X-ray viewing device which includes a series of masks on an elongated scroll. In a specific embodiment of the invention, the scroll is moved by a motor on a pair of feed rolls. The masks are sized to correspond with stock sizes of X-ray transparencies. The apparatus has a dimensional sensing mechanism which aligns a selected mask with a positioned transparency automatically in accordance with the sensed dimension. The transparencies are inserted into a film-holder. The widthwise sensing function is performed by a series of levers or fingers positioned to engage an edge of the film.
U.S. Pat. No. 4,637,150 to Geluk describes a system in which a cathode ray tube (CRT) is used as a light source and the light emitted by this source is modulated in accordance with the stored density of a transparency. This system is impractical due to the limited sizes and associated light intensity outputs of CRTs for this type of illuminator.
U.S. Pat. No. 4,908,876 to Deforest et al., describes, inter alia, a transparency viewer using projection lens to project a light source for backilluminating a transparency.
U.S. Pat. No. 5,313,726 to Yaniv et al., describes a transparency viewer in which a light source, mounted in a reflecting housing, is used to back-illuminate a transparency.
German Patent Application DE 33 31 762 A1 describes an array type electrochromatic illuminator in which back-lighting selectively illuminates portions of a viewing surface in response to the application of voltage to horizontal and vertical strip conductors on opposite faces of an electrochromatic material placed between the source of illumination and the viewing surface.
U.S. patent application Ser. No. 07/861,982, titled "Self Masking Transparency Viewing Apparatus" by Dan Inbar and Giora Teltsch, now U.S. Pat. No. 5,430,964, granted Jul. 11, 1995, and U.S. patent application Ser. No. 08/175,372, titled "Position Sensing Display Device" by Dan Inbar and Giora Teltsch, filed Mar. 31, 1994 and corresponding PCT Publications WO 91/10152, filed Dec. 28, 1990, and WO 93/01564, filed Jul. 11, 1991, the disclosures of which are incorporated herein by reference, disclose self-masking transparency viewing apparatus having a mask-pattern generating device which may be an electrically-controlled Liquid Crystal Array (LCA). In addition, there is provided a transparency detection means, such as optical sensors or micro switches. The optical sensors recognize optical properties, for example, attenuation of transparencies mounted and of unobstructed areas of the display area. In some embodiments the detection means determines the existence and locations of transparencies on the display surface, as well as the location of subimages within the transparencies. The detection data is transferred to a system control unit which drives the LCA to produce a complementary masking pattern in conformity with the displayed transparencies, masking other portions of the display area.
Back-illuminating a large LCA using direct illumination by a bank of fluorescent lamps, as shown in some prior art systems, has several drawbacks:
(a) the intensity of the back-illumination is typically limited to intensity levels in which the human eye is less sensitive to low-contrast details, due to limited area behind the display surface which is available for packing fluorescent lamps; PA1 (b) at least two LCA layers are required to provide good contrast at acceptable viewing angles, since large LCAs typically have low contrasts (especially at large viewing angles); the plurality of LCAs reduces the intensity of the back-illumination still further; PA1 (c) spatial uniformity is difficult to maintain when using a plurality of fluorescent lamps; PA1 (d) producing large, high-contrast, LCAs, in particular STN (super twisted nematic) and PDLCA (polymer dispersed liquid crystal array), at acceptable yields and cost is difficult; PA1 (e) producing very large LCAs, such as 14".times.17" is not known in the art, thus, a plurality of LCAs must be used to cover a large display size, such as 14".times.17", resulting in seams or dead spaces between the LCAs; seams are a drawback especially for horizontal mounting of large transparency sizes, such as 14".times.17", on viewboxes optimized for vertical mounting of 14".times.17" transparencies; PA1 (f) large LCAs are very expensive; PA1 (g) certain types of LCAs, such as active matrix, that allow high contrast and high complexity masking are unavailable in large sizes; PA1 (h) cooling the LCAs in prior systems is difficult; and PA1 (i) light recycling is complex and inefficient in systems which use large LCAs.
Rear projection displays of images are well known. In particular, video monitors using an LCA projector are known. Generally, an LCA containing an image modulates an intense collimated light source to produce an image carrying beam. The image carrying beam is then projected, using a lens, onto the back of a display surface. Such a system is described in the lecture notes of "Projection Displays", a lecture given by Alan Sobel at Bally's hotel in Las Vegas, Nev., on May 14, 1990 for the Society For Information Display and in the lecture notes of "Electronic Projection Displays", a lecture given by Frederic J. Kahn at the Washington State Convention Center, Seattle, Wash. on May 17, 1993, the disclosures of both of which are incorporated herein by reference.
It is known in the art, that when modulating a light beam using an LCA, the light beam is attenuated to a significant degree. This attenuation is due to the fact that LCAs modulate linearly polarized light and transforming regular light into linearly polarized light usually involves a loss of intensity of 50% or more. Typically, the polarizer absorbs the non-transmitted light, resulting in significant heat generation and possible degradation of the polarizer or LC (liquid crystal) material. In recent year, several types of relatively low-loss linear polarizers has been developed.
FIG. 1A shows a first such polarizer, as described in "A Polarization Transforming Optics for High Luminance LCA Projector", by S. Shikama, E. Toide and M. Kondo, in the proceedings of EURODISPLAY '90, the tenth international display research conference, the disclosure of which is incorporated herein by reference. An unpolarized light beam 20 is split by a polarizing beam splitter 22 into a first polarized light beam 28 and a second polarized light beam 34 which have perpendicular polarization axes. Beam 34 is reflected by a mirror 24 to pass through a half-wave plate 26 which rotates the polarization axis of beam 34 by 90.degree. to produce a beam 30. The polarization axes of beam 30 and beam 28 are parallel to each other and to the polarization axis of an input (to an LCA) polarizer 32. Thus, substantially all of the energy of light beam 20 passes through polarizer 32. Beam 30 is typically refracted to be parallel to beam 30 by additional optical elements (not shown).
FIG. 1B shows a second example of a low-loss polarizer, as described in "Polarization Conversion System LCD Projection", by A. J. S. M. De Vaan, A. H. J. Van De Brandt, R. A. M. Karsmakers, M. V. C. Stroomer and W. A. G. Timmers in the conference proceedings of EURODISPLAY '93, the 13th international display research conference, the disclosure of which is incorporated herein by reference. A polarizer 42 generally comprises a first prism portion 40 and a second prism portion 44 which are separated by thin birefringent layer 46, such as a birefringent adhesive. A light beam 48 entering prism 48 is refracted towards layer 46, which seoarates beam 48 into a first polarized beam 50, which is reflected back into prism 40, and into a second polarized beam 52, which is transmitted into prism 44. Beams 50 and 52 have perpendicular polarization axes. When beam 50 (and 52) exit prisms 40 (and 44), it is refracted back by an angle equal to its entrance angle. The polarization axis of beam 52 is rotated 90.degree. by a half-wave plate 54 located on or near the exit of prism 44. Thus, beam 52 and beam 50 have parallel polarization axes.